Field of the Invention
The present invention relates generally to a turbine rotor blade of a gas turbine engine, and more specifically to a turbine rotor blade having a spar and shell construction that can be easily assembled and disassembled.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, a hot gas stream is passed through a turbine to drive a fan or an electric generator to power an aircraft in an aero gas turbine engine or to produce electrical power in the case of an industrial gas turbine engine. An efficiency of the engine can be increased by passing a higher temperature gas into the turbine. However, the turbine inlet temperature (TIT) is limited to the material properties of the turbine such as the turbine stator vanes and the rotor blades. Internal cooling of the vanes and blades of the turbine is done to allow for gas stream temperatures higher than the melting point of the airfoils. A typical turbine airfoil (blade or vane) is made from a nickel alloy which has high temperature resistance as well as high strength for use in a turbine. A nickel alloy rotor blade is formed as a single solid piece with internal cooling passages formed from an investment casting process. Stator vanes are also formed from casting a nickel alloy but with impingement cooling inserts added after the vane is cast. However, turbine inlet temperatures have reached a limit using these nickel alloy materials with airfoil cooling.
Recently the turbine rotor blade is formed from a spar and shell construction in which a shell is secured to a spar to form the rotor blade. With the spar and shell construction, the shell can be made from a different material than the spar. Thus, the shell can be formed from a higher temperature resistant material than that of the spar. The spar takes all the load from the shell so that the shell can be made mostly for high temperature resistance than for high stress resistance. With the spar and shell construction, the shell can be formed from a ceramic material or even a refractory metal such as molybdenum. One major problem with the spar and shell constructed turbine rotor blade is in the assembly or disassembly of the shell from the spar and of retaining the shell to the spar against the high centrifugal loads due to the rotation of the rotor blade.
Single crystal metals have been used for turbine rotor blades because of their high stiffness against centrifugal loads. To continue to increase the turbine inlet temperature beyond the current single crystal material capability different materials systems needs to be utilized. Ceramic materials and refractory alloys have higher use temperatures but do not have the tensile strength to be self-supporting at the rotational speeds and temperatures that are desirable. These materials could be used as a closeout for the airfoil if they could be kept in a compressive state or in a partially compressive and mild tensile state. This can be achieved by restraining the shell at the outer edge of the airfoil via a shelf that is attached to the tensile member called the spar which is internal to the shell. There is a practical limit to the speed and consequently the pull exerted on the spar because the entire load of the shell must be taken out at the tip and then this load transitioned inboard to the blade attachment.